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The Geometry of the Group of Symplectic Diffeomorphisms

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The Geometry of the Group of Symplectic Diffeomorphisms The geometry of the group of symplectic di®eomorphisms Leonid Polterovich October 15, 2007 Preface The group of Hamiltonian di®eomorphisms Ham(M; ­) of a symplectic manifold (M; ­) plays a fundamental role both in geometry and clas- sical mechanics. For a geometer, at least under some assumptions on the manifold M, this is just the connected component of the identity in the group of all symplectic di®eomorphisms. From the viewpoint of mechanics, Ham(M; ­) is the group of all admissible motions. What is the minimal amount of energy required in order to generate a given Hamiltonian di®eomorphism f? An attempt to formalize and answer this natural question has led H. Hofer [H1] (1990) to a remarkable dis- covery. It turns out that the solution of this variational problem can be interpreted as a geometric quantity, namely as the distance between f and the identity transformation. Moreover this distance is associated to a canonical biinvariant metric on Ham(M; ­). Since Hofer's work this new geometry has been intensively studied in the framework of modern symplectic topology. In the present book I will describe some of these developments. Hofer's geometry enables us to study various notions and prob- lems which come from the familiar ¯nite dimensional geometry in the context of the group of Hamiltonian di®eomorphisms. They turn out to be very di®erent from the usual circle of problems considered in sym- plectic topology and thus extend signi¯cantly our vision of the sym- plectic world. Is the diameter of Ham(M; ­) ¯nite or in¯nite? What are the minimal geodesics? How can one ¯nd the length spectrum? In general, these questions are still open. However some partial answers do exist and will be discussed below. There is one more, to my taste even more important reason why it is useful to have a canonical geometry on the group of Hamiltonian i ii di®eomorphisms. Consider a time dependent vector ¯eld »t; t 2 R on a manifold M. The ordinary di®erential equation x_ = »(x; t) on M de¯nes a flow ft : M ! M which takes any initial condition x(0) to the solution x(t) at time t. The trajectories of the flow form a complicated system of curves on the manifold. Usually, in order to understand the dynamics, one should travel along the manifold and thoroughly study behaviour of the trajectories in di®erent regions. Let us change the point of view and note that our flow can be interpreted as a simple geometric object - a single curve t ! ft on the group of all di®eomorphisms of the manifold. One may hope that geometric properties of this curve reflect the dynamics, and thus complicated dynamical phenomena can be studied by purely geometric tools. Of course, the price we pay is that the ambient space - the group of di®eomorphisms - is in¯nite-dimensional. Moreover, we face a serious di±culty: in general there are no canonical tools to perform geometric measurements on this group. Remarkably, Hofer's metric provides such a tool for systems of classical mechanics. In Chapters 8 and 11 we will see some situations where this ideology turns out to be successful. As it often happens with very young and fast developing areas of mathematics, the proofs of various elegant statements in Hofer's geometry are technical and complicated. Thus I selected the simplest non-trivial versions of the main phenomena (according to my taste, of course) without making an attempt to present them in the most general form. For the same reason many technicalities are omitted. Though formally speaking this book does not require a special back- ground in symplectic topology (at least necessary de¯nitions and for- mulations are given), the reader is cordially invited to consult two remarkable introductory texts [HZ] and [MS]. Both of them contain chapters on the geometry of the group of Hamiltonian di®eomor- phisms. I have tried to minimize the overlaps. The book contains a number of exercises which presumably will help the reader to get into the subject. This book arose from two sources. The ¯rst one is lectures given to graduate students, namely mini-courses at Universities of Freiburg iii and Warwick, and a Nachdiplom-course at ETH, ZÄurich. The second source is the expository article [P8] which contains a brief outline of the material presented below. Let me introduce briefly the main characters of the book. A dif- feomorphism f of a symplectic manifold (M; ­) is called Hamiltonian if it can be included into a compactly supported Hamiltonian flow fftg with f0 = 1l and f1 = f. Such a flow is de¯ned by a Hamiltonian function F : M £ [0; 1] ! R. In the language of classical mechanics F is the energy of a mechanical motion described by fftg. We interpret the total energy of the flow as a length of the corresponding path of di®eomorphisms: Z 1 lengthfftg = max F (x; t) ¡ min F (x; t)dt: 0 x2M x2M De¯ne a function ½ : Ham(M; ­) £ Ham(M; ­) ! R by ½(Á; Ã) = inf lengthfftg; where the in¯mum is taken over all Hamiltonian flows fftg which generate the Hamiltonian di®eomorphism f = Áá1. It is easy to see that ½ is non-negative, symmetric, vanishes on the diagonal and satis¯es the triangle inequality. Moreover, ½ is biinvariant with respect to the group structure of Ham(M; ­). In other words, ½ is a biinvariant pseudo-distance. It is a deep fact that ½ is a genuine distance function, that is ½(Á; Ã) is strictly positive for Á 6= Ã. The metric ½ is called Hofer's metric. The group Ham(M; ­) and Hofer's pseudo-distance ½ are intro- duced in Chapters 1 and 2 respectively. In Chapter 3 we prove that ½ is a genuine metric in the case when M is the standard symplectic linear space R2n. Our approach is based on Gromov's theory of holomor- phic discs with Lagrangian boundary conditions which is presented in Chapter 4. Afterwards we turn to the study of basic geometric invariants of Ham(M; ­). There is a (still open!) conjecture that the diameter of iv Ham(M; ­) is in¯nite. In Chapters 5-7 we prove this conjecture for closed surfaces. In Chapter 8 we discuss the concept of the growth of a one- parameter subgroup fftg of Ham(M; ­) which reflects the asymptotic behaviour of the function ½(1l; ft) when t ! 1. We present a link be- tween the growth and the dynamics of fftg in the context of invariant tori of classical mechanics. In many interesting situations the space Ham(M; ­) has compli- cated topology, and in particular a non-trivial fundamental group. For an element γ 2 ¼1(Ham(M; ­)) set º(γ) = inf lengthfftg; where the in¯mum is taken over all loops fftg of Hamiltonian di®eomorphisms (that is periodic Hamiltonian flows) which represent γ. The set ¯ ¯ fº(γ) ¯ γ 2 ¼1(Ham(M; ­))g is called the length spectrum of Ham(M; ­). In Chapter 9 we present an approach to the length spectrum which is based on the theory of symplectic ¯brations. An important ingredient of this approach is Gromov's theory of pseudo-holomorphic curves which is discussed in Chapter 10. In Chapter 11 we give an application of our results on the length spectrum to the classical ergodic theory. In Chapters 12 and 13 we develop two di®erent approaches to the theory of geodesics on Ham(M; ­). One of them is elementary, while the other requires a powerful machinery of Floer homology. Chapter 13 suggests to the reader a brief visit to Floer homology. Finally, in Chapter 14 we deal with non-Hamiltonian symplectic di®eomorphisms, which appear naturally in Hofer's geometry as isome- tries of Ham(M; ­). In addition, we formulate and discuss the famous flux conjecture which states that the group Ham(M; ­) is closed in the group of all symplectic di®eomorphisms endowed with the C1- topology. v Acknowledgments. I cordially thank Meike Akveld for her in- dispensable help in typing the preliminary version of the manuscript, preparing the pictures and enormous editorial work. I am very grateful to Paul Biran and Karl Friedrich Siburg for their detailed comments on the manuscript and for improving the presentation. I am indebted to Rami Aizenbud, Dima Gourevitch, Misha Entov, Osya Polterovich and Zeev Rudnick for pointing out a number of inaccuracies in the pre- liminary version of the book. The book was written during my stay at ETH, Zurich in the academic year 1997-1998, and during my visits to IHES, Bures-sur-Yvette in 1998 and 1999. I thank both institutions for the excellent research atmosphere. vi Contents 1 Introducing the group 1 1.1 The origins of Hamiltonian di®eomorphisms . 1 1.2 Flows and paths of di®eomorphisms . 4 1.3 Classical mechanics . 5 1.4 The group of Hamiltonian di®eomorphisms . 7 1.5 Algebraic properties of Ham(M; ­) . 13 2 Introducing the geometry 15 2.1 A variational problem . 15 2.2 Biinvariant geometries on Ham(M; ­) . 16 2.3 The choice of the norm: Lp vs. L1 . 18 2.4 The concept of displacement energy . 19 3 Lagrangian submanifolds 25 3.1 De¯nitions and examples . 25 3.2 The Liouville class . 27 3.3 Estimating the displacement energy . 31 4 The @¹-equation 35 4.1 Introducing the @¹-operator . 35 4.2 The boundary value problem . 37 4.3 An application to the Liouville class . 39 4.4 An example . 41 5 Linearization 45 5.1 The space of periodic Hamiltonians . 45 5.2 Regularization .
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